The present invention relates to a multi-spindle synchronous drive unit for synchronously driving a plurality of spindles at arbitrary speeds, respectively and a gear cutting machine such as a gear shaving machine employing the multi-spindle synchronous drive unit.
For example, in a known gear shaving machine shown in FIG. 1, master gears 33 and 34 having a gear ratio identical with that of a gear cutter 31 and a gear blank 32 to be machined are, respectively, secured to spindles 35 and 36 so as to be engaged with each other such that the gear cutter 31 and the gear blank 32 are, respectively, mounted on the spindles 35 and 36. The spindle 35 having the gear cutter 31 mounted thereon is driven by a motor 37 and the gear cutter 31 and the gear blank 32 are synchronously driven so as to be rotated at an identical peripheral speed.
Meanwhile, in order to cut the gear blank 32 using the gear cutter 31, the spindles 35 and 36 are subjected to elastic torsional deformation at the time of machining of the gear blank 32 such that an elastic restoring force of the spindles 35 and 36 caused by the elastic torsional deformation urges the gear cutter 31 to a position yielding a predetermined depth of cut.
Furthermore, FIG. 2 shows a prior art synchronous drive unit in which a plurality of spindles are rotatably driven at a fixed speed ratio. In FIG. 2, motors 41 and 42 for driving spindles (not shown), respectively are provided and encoders 43 and 44 are, respectively, connected to the motors 41 and 42 such that feedback control for accurately rotating the motors 41 and 42 at preset speeds, respectively is performed by an NC (numerical control) unit 45. In addition, a gear shaving machine employing this prior art synchronous drive unit is also proposed.
However, in the known mechanical synchronous method employing the master gears as referred to above, since combination of the master gears is restricted, shaving of a gear blank having an arbitrary number of teeth cannot be performed and the depth of cut is fixed. Furthermore, an inconvenience is incurred in that it is impossible to control the cutting feed rate in order to minimize machining time.
Meanwhile, in the above mentioned prior art synchronous drive unit in which rotational speeds of the spindles are controlled by the NC unit, the speeed ratio of the sprindles can be set arbitrarily. However, the speed ratio is calculated digitally so as to determine a control value in the prior art synchronous drive unit. Therefore, in the case where the speed ratio is undividable, for example, 1:3, fraction is generated in the control value, thereby inevitably resulting in inaccurate positioning of the spindles. In the case of gear shaving, the number of teeth of the gear often assumes a prime number or odd number, the speed ratio of the gear to the gear cutter is often undividable and thus, it is difficult to perform shaving of the gear at high precision. Meanwhile, it is necessary to set the cutting feed rate in accordance with diameter of the gear blank. However, the cutting feed rate is manually reset based on experience by using the NC unit when the diameter of the gear blank has changed, which is time-consuming and lowers machining efficiency.